Purified B cell precursors (BCP) (CD10+ CD19+ surface-membrane (s)Ig-cells) isolated from human fetal bone marrow (BM) were cultured with various cytokines, in the presence or absence of a fibroblastic stromal cell layer derived from adult human BM. We demonstrated that IL-7, IL-3, and stem-cell factor (SCF) participate in inducing low magnitude BCP proliferation in the absence of stroma. Addition of either IL-4, IFN (alpha and gamma), or TGF beta, resulted in significant inhibition of proliferation. Strikingly, BCP proliferated at remarkably higher levels when cultured on BM stromal cells, and this effect was further enhanced by exogenously supplied IL-7. Proliferating cells were mostly CD20+, and included both c mu- and c mu+ cells. Furthermore, BCP proliferated in response to anti CD40 antibody presented by Fc gamma RII-transfected murine fibroblastic Ltk- cells (CD40 system) (Banchereau et al. 1991), demonstrating a functional role for CD40 in B cell ontogeny. However, this effect was shown to require a second signal, which could be specifically provided by IL-3 among a panel of cytokines examined. Finally, although suggestive of BCP maturation, the culture systems examined did not permit the transition to mature B cells (sIgM+ sIgD+).